1. Field of the Disclosure
This disclosure is directed to a light-emitting diode (LED) lamp, and more particularly to an apparatus and method for more efficiently driving an LED lamp.
2. Related Art
An LED lamp is a type of solid state lighting (SSL) that uses one or more LEDs as a light source. LED lamps are usually constructed with one or more clusters of LEDs in a suitable housing. FIG. 1A shows a configuration of a conventional LED lamp 100. The LED lamp 100 includes a voltage source 110, a rectifier 120, a current source 130 and an LED cluster 140. The LED cluster 140 typically includes a plurality of LEDs 140A to 140N connected in series to form an LED string coupled between the current source 130 and a ground 150. The LED cluster 140 may include more than one LED string coupled in parallel between the current source 130 and the ground 150. The voltage source 110 may be an AC voltage source. The AC voltage from the voltage source 110 is converted to a DC voltage by the rectifier 120 and provided as an input voltage VINPUT to the LED cluster 140. The current source 120 may be configured to impose a maximum current IMAX of a current ILED flowing through the LED cluster 140.
FIG. 1B is a graph showing changes in the current ILED in response to a sinusoidal input voltage VINPUT. Initially at time t0, the input voltage VINPUT and the current ILED is the lowest (i.e., zero) and the LED cluster 140 may stay turned off until the input voltage VINPUT rises and reaches a sufficient potential level (i.e., a threshold level VTH) at which time the LED cluster 140 is turned on and the current ILED begins to flow therethrough at time t1. As the input voltage VINPUT further increases, the current ILED also increases until it reaches the maximum current IMAX set by the current source 130 at time t2 (The input voltage VINPUT at the time t2 is referred to as a maximum voltage VMAX). Upon reaching the maximum current IMAX, the current ILED stays substantially the same even though the input voltage VINPUT rises over the maximum voltage VMax. After reaching the peak of sinusoidal curve, the input voltage VINPUT falls but the current ILED stays at the maximum current IMAX until the input voltage VINPUT further falls below the maximum voltage VMAX at time t3. After passing the time t3, the current ILED begins to decrease as the input voltage VINPUT further decreases from the maximum voltage VMAX. The current ILED is then discontinued when the input voltage VINPUT falls below the threshold level VTH at time t4. This pattern is repeated in the subsequent input voltage cycles.
The LED lamp 100 shown in FIG. 1A, however, suffers various drawbacks, some of which may contribute to inefficient power consumption. For example, between the times t2 and t3, the LED cluster 140 cannot convert the input voltage VINPUT higher than the maximum voltage VMAX to light and the excessive energy is instead converted to heat. Furthermore, the LED cluster 140 may be turned on only for the period between the times t1 and t4, i.e., when the input voltage VINPUT is higher than the threshold level VTH. Thus, the LED lamp 100 suffers a relatively short duty cycle compared to the input voltage cycle. The duty cycle may be even further shortened when LED cluster 140 has a higher threshold level VTH.
Accordingly, there is a need for an improved LED lamp configuration and power scheme to increase the energy efficiency and improve the light-generating operation.